The transport by diffusion of Hg(II) and Hg(O) through a barrier of concrete or bentonite-enhanced sand was examined under aerobic conditions. Sand was used as a reference system parallel to the two systems. Speciation of mercury was performed with a purge and trap method, where dissolved Hg(O) was purged with nitrogen gas from the sample, through a trap for volatile oxidized mercury species and finally trapped in an oxidative solution. The apparent diffusion coefficient (from Fick's second law of diffusion) for oxidized mercury was 1 x 10(-14) m(2)/s in Standard Portland concrete and 4 x 10(-13) m(2)/s in quartz sand. The diffusion of Hg(O) seemed to be faster than for Hg(II), Hg(O) was however oxidized to Hg(H) under aerobic conditions, and after 45 months only 1-10% of the total mercury concentration was Hg(O). (c) 2006 Elsevier B.V. All rights reserved.